Production of spheroidized particles



p 1966 D. J. N. HOFFMAN ETAL 39 9 PRODUCTION 0F SPHEROIDIZED PARTICLESFiled July 25, 1963 United States Patent 3,272,615 PRODUCTION OFSPHEROIDIZED PARTICLES Daniel l. N. Hoffman and Thomas ll. Beeton,Pretoria, Transvaal, Republic of South Africa, assignors to SouthAfrican Iron and Steel Industrial Corporation Limited Filed July 25,1963, Ser. No. 297,635 Claims priority, application Republic of SouthAfrica, Aug. 1, 1962, 626,266 2 Elaims. (Cl. 75.5)

This invention relates to the production of spheroidized particles, andmore particularly spheroidized ferrosilicon particles.

The specification of our US. Pat. 3,015,852, discloses a method ofspheroidizing irregularly shaped solid particles, such as ferro-siliconparticles, to form smooth spheroidized solid particles by passing theparticles through a high temperature flame in which they are melted atleast at their surfaces, and thereafter cooling the thus treatedparticles.

More particularly, a method of spheroidizing irregularly shapedparticles includes the steps of passing a gas containing free oxygen inat least the proportion contained in air into an inner passage of aflame-producing nozzle; introducing a combustible gas through an annularpassage in the nozzle surrounding the inner passage, thus producing aflame having a reducing zone at least towards its perimeter; feeding theirregularly shaped particles to be spheroidized into the inner passage;causing the particles to pass through the flame and its reducing zonethereby melting them at least at their surfaces; and allowing the thusspheroidized particles to enter a cooling zone.

With the method defined in the previous paragraph, oxidizing gas isblown into the center of the flame, and a combustible gas is supplied toproduce a surrounding reducing zone at least towards the periphery ofthe flame. Particles to be spheroidized pass from the oxidizing centerof the flame through the reducing zone upon leaving the flame and, as aresult, too extensive oxidation of the particles is prevented. Aso-called inverse flame is produced.

It is believed that the flame characteristics can be improvide by mixinga small proportion of combustible gas with the oxidizing gas introducedinto the inner passage of the flame-producing nozzle.

Preferably, the irregularly shaped particles to be spheroidized arepassed through a downwardly directed pencil-shaped flame.

irregularly shaped ferro-silicon particles containing from about tosilicon, and preferably from about 12 to 17% silicon, are very suitableas the initial material. Small amounts of other alloying constituents,such as, for example, copper or aluminum, which may have a beneficialeffect on the spheroidizing, corrosion resistance, or other qualities ofthe particles, may be present or incorporated therein. The initialmaterial may, for example, be mechanically ground and then subjected tothe spheroidizing treatment.

It is an object of the present invention to provide improvedspheroidized particles which are superior to the particles obtained inaccordance with the method disclosed in our US. Patent 3,015,852.

According to the invention a method of spheroidizing irregularly shapedparticles includes the steps of providing a high temperature flame;imparting a swirling motion to the irregularly shaped particles; passingthe swirling particles through the flame; and allowing the particles topass from the flame into a cooling zone.

More particularly, a method of spheroidizing irregularly shapedparticles includes the steps of discharging a gas containing free oxygenin at least the proportion contained in air from an inner passage of aflame-producing nozzle; discharging a combustible gas through an annularpassage in the nozzle surrounding the inner passage, thus producing aflame having a reducing zone at least towards its perimeter; feeding theirregularly shaped particles to be spheroidized into the inner passage;imparting a swirling motion to particles issuing from the innerpass-age; causing the swirling particles to pass through the flame andits reducing zone thereby melting them at least at their surfaces; andallowing the particles to pass from the flame into a cooling zone.

With the method defined in the previous paragraph an inverse flame isproduced.

Preferably, a substantially circular swirling motion is imparted to theparticles.

The swirling motion may conveniently be imparted to the particles byimparting a swirling motion to gas issuing from the flame-producingnozzle.

Preferably, a swirling motion is imparted to combustible gas issuingfrom the annular passage of the nozzle. The surrounding combustible gastransmits its swirling motion to the oxygen containing gas and theparticles to be spheroidized which issue from the inner passage of thenozzle.

An inverse flame as described above is normally of the diffusioncontrolled type which tends to have an inner hollow zone in which nocombustion occurs, the oxidizing zone of the flame surrounding the innerzone. As a result of their swirling motion, the particles to bespheroidized are thrown outwardly by centrifugal force from the coldinner zone into a hot zone of the flame.

It will be appreciated that the degree of swirl of particles should beregulated so as to be suificient to maintain them in the hot zone of theflame for an adaquate period of time. In the case of a downwardlydirected flame such as is preferably employed, some of the particlesmight miss the hot zone of the flame by falling more or less verticallydownwards, if the degree of particle swirl is too low. On the otherhand, if the degree of particle swirl is too great, some of theparticles will pass too rapidly through the hot zone of the flame.

It has been found that suitable swirling of the particles to bespheroidized not only improves the efllciency of operation, but alsoproduces improved spheroidizing of the particles.

The amount of swirl imparted to gases and particles issuing from theinverse flarne-producing nozzle can be adjusted to some extent byspacing the mouth of the inner passage from the nozzle mouth and varyingthe distance between the mouth of the flame-producing nozzle and themouth of the inner passage.

Preferably, the oxygen containing gas issuing from the inner passage andthe combustible gas issuing from the surrounding annular passage of thenozzle have closely similar exit velocities. This assists in maintaininga stable flame since it minimizes the formation of eddies at theinterface of the two gases.

It has been found that the spheroidizing operation and also the qualityof the spheroidized product in the case of oxidizable particulateinitial material, such as ferro-silicon particles, is improved bysetting up an additional envelope of reducing gas around the inverseflame.

The additional envelope of reducing gas should be dis tributed veryevenly all around the flame to ensure that the hot zone of the flameproper is completely surrounded by an essentially reducing zone of lowertemperature. The additional reducing gas should preferably issue fromthe nozzle at a high velocity. This materially assists in preventingfine particles of material undergoing spheroidizing from escaping fromthe flame too quickly.

No substantial swirl need be imparted to the additional envelope ofreducing gas, although it may be done if necessary.

According to another aspect of the invention a flameproducing nozzle forspheroidizing irregularly shaped particles includes an inner passage fordischarging oxygen containing gas and irregularly shaped particles; anannular passage for discharging combustible material, the annularpassage surrounding the inner passage; and a tangential inlet into theannular passage.

The flame-producing nozzle may include a supply passage for oxygencontaining gas, communicating with the inner passage; and a particlefeed passage located within the supply passage for oxygen containing gasand having an outlet directed towards the interior of the inner passage.

The supply passage for oxygen containing gas may be connected to theinner passage through a venturi tube, the outlet of the particle feedpassage being located at or near the restricted zone of the venturitube.

The outlet from the inner passage may be spaced from the mouth of thenozzle.

The flame-producing nozzle may include an additional annular passage fordischarging reducing gas, the additional annular pasage surrounding theannular passage for combustible gas.

A preferred embodiment of the invention will now be described by way ofexample with reference to the accompanying drawings in which:

FIGURE 1 is a sectional view of a downwardly directed flame-producingnozzle according to the invention.

FIGURE 2 is a section on line IIII in FIGURE 1.

FIGURE 3 is a section on line IIIIII in FIGURE 1.

FIGURE 4 is a section on line IVIV in FIGURE 1.

Irregularly shaped particles of ferro-silicon alloy to be spheroidizedare fed through feed hopper 1 into particle feed pipe 2, the outlet 3 ofwhich is directed towards the interior of inner discharge tube 4.Pre-heated air is introduced through inlet pipe 5 into supply tube 6which surrounds particle feed pipe 2 and which communicates with innerdischarge tube 4 through venturi tube 7.

As can be seen from the drawing, outlet 3 of particle feed pipe 2 islocated near the restricted zone 7a of venturi tube 7. With thisarrangement, flow of air through venturi tube 7 induces a suction effectin particle feed pipe 2, the suction eifect assisting in introducingirregularly shaped particles into inner discharge tube 4. Usually, onlya very small quantity of atmospheric air is inducted into particle feedpipe 2 through hopper 1, as under normal conditions feed pipe 2 carriesa full load of irregularly shaped particles to be spheroidized.

Air is introduced from inlet 5 into supply tube 6 through verticalapertures 8 so that the air flows straight down supply tube 6 aroundparticle feed pipe 2 without any substantial swirling action. It will beappreciated that irregularly shaped particles entering venturi tube 7from particle feed pipe 2 are dispersed in air entering venturi tube 7from supply tube 6. A mixture of pre heated air and irregularly shapedparticles is discharged from outlet 9 of inner tube 4. Outlet 9 isspaced from nozzle mouth 10.

Coke oven gas or other suitable combustible fuel gas is introducedtangentially by means of inlet pipe 11 into chamber 12 which is locatedconcentrically around inner discharge tube 4 and converges towardsnozzle mouth 10. The tangential introduction of the fuel gas causes itto pass down chamber 12 around inner tube 4 with a pronounced swirlingmotion to induce at or near nozzle mouth a swirl in the air streamissuing from central tube 4 as well as in irregularly shaped particlesdispersed in the air stream.

The air and the fuel gas issuing from nozzle mouth 10 produces a pencilshaped, downwardly directed inverse flame 13 with an oxidizing zone 14in which the highest temperature in the flame occurs, and a surroundingreducing zone 15 at least towards the periphery of flame 13.

Oxidizing zone 14 is located around a cold air-containing inner zone 20in which no combustion occurs. Irregularly shaped particles issue fromnozzle mouth 10 into cold inner zone 20 and as a result of theirswirling motion, the particles are thrown out of inner zone 20 into thehot oxidizing zone 14 where they are melted at least at their surfacesbefore passing through reducing zone 15 and out of the flame.

Since the particles pass through reducing zone 15 upon leaving flame 13,too extensive an oxidation of the particles is prevented.

It may happen that the cold inner zone 20 extends down to the bottom offlame 13. If the degree of particle swirl is too low, some of theparticles might miss the hot zone 14 of the flame by falling more orless vertically downwards. On the other hand, if the degree of particleswirl is too great, some of the particles will pass too rapidly throughhot zone 14 of the flame. The degree of swirl should be sufficient tomaintain the particles in hot zone :14 of the flame for an adequateperiod of time to permit them to be melted at least at their surfaces.

The nozzle is so shaped and the air and the fuel gas introduced into thenozzle at such pressures that the air and the fuel gas are discharged atsubstantially similar exit velocities. This assists in maintaining astable flame since it minimizes the formation of eddies at the interfacebetween the air and the fuel gas. The exit velocities can be adjustedwithin limits by raising and lowering outlet 9 of inner discharge tube 4in relation to nozzle mouth 10. This can be effected by adjusting theposition of upper nozzle portion A relative to lower nozzle portion B byadjusting the extent to which upper portion A is screwed into lowerportion B at screw-threaded engagement 16.

The degree of particle swirl will depend on the exit velocity ofcombustible gas issuing from annular chamber 12. The degree of swirl canbe adjusted to some extent by adjustment of the position of outlet 9 ofinner discharge tube 4 in relation to nozzle mouth 10.

The nozzle also includes annular chamber 17 in communication with outerannular discharge passage 18. A reducing gas, such as coke oven gas,introduced into chamber 17 through inlet 19, is discharged at a highvelocity through outer discharge passage '18 to form an additionalenvelope 15a of reducing gas which completely envelopes flame 13. Thereducing envelope 15a is at a lower temperature than flame 13. The highvelocity of reducing envelope 15a helps to prevent the finer particlesof material undergoing spheroidizing from escaping too quickly from theflame.

As can be seen from the drawings, particle feed pipe 2, supply tube 6,venturi tube 7, inner discharge tube 4, annular chamber 12 and outerannular passage 18 are all located coaxially.

Lower nozzle portion B is cooled by means of water introduced throughpipe 21 into cooling jacket 22. Further cooling jackets may be providedif necessary.

Upon passing through flame 13, the particles to be spheroidized aremelted at least at their surfaces and assume spheroidal shapes.

After passing out of flame 13, the particles are allowed to cool andsolidify. As shown in FIGURE 1, the flameproducing nozzle is mounted onthe upper end of cooling chamber 23 and is arranged to direct flame 13downwardly into chamber 23, which provides a cooling zone. Annular inlet24 is provided in the top of chamber 23 for directing a curtain ofcooling medium down the inner periphery 25 of chamber 23. Cooling mediummay also be introduced tangentially into chamber 23 at one or morelevels along the height of chamber 23 through one or more peripheralinlets (not shown).

Solidified spheroidized particles may be discharged from chamber 2 3into a suitable receptacle (not shown). Further cooling means, such as,for example, a heat exchanger, may be provided.

Wet or dry cooling and collection or separation of spheroidizedparticles may be used as described fully in our -U.S. Patent No.3,015,852, in which the cooling chamber is referred to as a shaftfurnace because of the flame therein.

Cooling chamber 23 is advantageously provided with automatic pressurecontrol means (not shown). This is of particular importance in casesWhere the proper dispersion of the particles to be spheroidized in theair stream is dependent on the maintenance of a certain amount ofsuction on the particle feed pipe 2.

The spheroidized particles produced in accordance with the presentinvention are characterized by a particular regularity of shape and agreat smoothness of surface, substantially Without angular corners.

The process is particularly suitable for the production of particleshaving a size distribution range below 200 mesh. Alloy particles andparticularly ferro-silicon particles of a particle size range below 250mesh, preferably below 270 mesh, can be spheroidized with good results.

It will be appreciated that many variations in detail are possiblewithout departing from the scope of the invention as defined in theappended claims.

Instead of air, any suitable gas containing free oxygen in at least theproportion contained in air may 'be discharged from inner passage 4. Asmall proportion of combustible gas, such as coke oven gas, producergas, or water gas, may be mixed with the oxygen containing gas toimprove the flame characteristics. Any suitable combustible fuel gasother than coke oven gas may be discharged from annular passage 12.Similarly, any suitable reducing gas other than coke oven gas may bedischarged from outer annular passage 18.

Also, any suitable cooling chamber or other suitable cooling arrangementmay be provided.

We claim:

1. In a method of spheroidizing irregularly shaped particles in which agas containing free oxygen in at least the proportion contained in airis discharged through an inner passage of a flame-producing nozzle intocontact with a combustible gas introduced through an annular passage inthe nozzle surrounding the inner passage to thus produce a flame havingan inner oxidizing zone and a reducing zone at least towards itsperimeter; the irregularly shaped partieles to be spheroidized being fedinto the inner passage and axially therealong, the improvementcomprising discharging the combustible gas tangentially into saidannular passage along a helical path around said inner passage toentrain the particles issuing from the inner passage and cause saidparticles to undergo a swirling motion as they leave said passage topass through the flame and its reducing zone thereby melting theparticles at least at their surfaces; the oxygencontaining gas beingdischarged from the inner passage at an exit velocity substantially thesame as the exit velocity of the combustible gas issuing from thesurrounding annular passage; and cooling the particles as they pass fromthe flame.

2. A method as claimed in claim 1, comprising forming an additionalenvelope of reducing gas around the flame.

References Cited by the Examiner UNITED STATES PATENTS 2,451,546 10/1948 Forton .5 2,530,345 11/1950 Watts 26412 2,675,295 4/ 1954 Daniels75-26 3,015,852 1/1962 Hoffman et al. -.5 3,041,672 7/1962 Lyle 264-103,059,860 10/1962 Hohn 239423 3,062,638 11/ 1962 Culbertson et al 75-.53,093,315 6/ 1963 Tachiki et al 239424 OTHER REFERENCES Article inJournal of Metals, January 1959, pp. 40-42. Tyler, Plasma for ExtractiveMetallurgy, Journal of Metals, January 1961, pp. 51-54.

DAVID L. RECK, Primary Examiner.

HYLAND BIZOT, N. F. MARKVA, Assistant Examiners.

1. IN A METHOD FOR SPEROIDIZING IRREGULARLY SHAPED PARTICLES IN WHICH AGAS CONTAINING FREE OXYGEN IN AT LEAST THE PROPORTION CONTAINED IN AIRIS DISCHARGED THROUGH AN INNER PASSAGE OF A FLAME-PRODUCING NOZZLE INTOCONTACT WITH A COMBUSTIBLE GAS INTRODUCED THROUGH AN ANNULAR PASSAGE INTHE NOZZLE SURROUNDING THE INNER PASSAGE TO THUS PRODUCE A FLAME HAVINGAN INNER OXIDIZING ZONE AND A REDUCING ZONE AT LEAST TOWARDS ITSPERIMETER; THE IRREGULARLY SHAPED PARTICLES TO BE SPHEROIDIZED BEING FEDINTO THE INNER PASSAGE AND AXIALLY THEREALONG, THE IMPROVEMENTCOMPRISING DISCHARGING THE COMBUSTIBLE GAS TANGENTIALLY INTO SAIDANNULAR PASSAGE ALONG A HELICAL PATH AROUND SAID INNER PASSAGE TOENTRAIN THE PARTICLES ISSUING FROM THE INNER PASSAGE AND CAUSE SAIDPARTICLES TO UNDERGO A SWIRLING MOTION AS THEY LEAVE SAID PASSAGE TOPASS THROUGH THE FLAME AND ITS REDUCING ZONE THEREBY MELTING THEPARTICLES AT LEAST AT THEIR SURFACES; THE OXYGENCONTAINING GAS BEINGDISCHARGED FROM THE INNER PASSAGE AT AN EXIT VELOCITY SUBSTANTIALLY THESAME AS THE EXIT VELOCITY OF THE COMBUSTIBLE GAS ISSUING FROM THESURROUNDING ANNULAR PASSAGE; AND COOLING THE PARTICLES AS THEY PASS FROMTHE FLAME.